(1) Field of the Invention
The present invention relates to a tuner circuit for use in a high-frequency circuit in a television receiver, video tape recorder or the like, and more particularly to a tuner circuit including circuit which uses a switching diode to change over receiving bands.
(2) Description of the Prior Art
A tuner circuit used in a television receiver or the like selects a signal belonging to a desired channel from television signals of VHF or UHF bands received by an antenna so as to convert the selected signal into its intermediate frequency signal to be input to a next step or an intermediate-frequency amplifier for video signal.
Most of recent tuner circuits are of electronic tuning type tuners in which coils in a tuning circuit are changed over by switching diodes while adjustment of capacitance of a capacitor in the tuning circuit is performed with a capacitance variable diode.
FIG. 1 is a circuit diagram showing a prior art example of an input tuning circuit in such a tuner circuit. The input tuning circuit is connected to a preceding filter circuit (not shown) with a coupling capacitor C1 and is connected to a next high-frequency amplifying circuit (not shown) with another coupling capacitor C2.
An output of coupling capacitor C1 is bifurcated into the earth side and the hot side. The earth side is connected with a series circuit of an earth side low-band coil L1 and an earth side high-band coil L2 while the hot side is connected with a series circuit of a hot side low-band coil L3 and a hot side high-band coil L4.
An earth side terminal of earth side high-band coil L2 is connected to a power supply terminal BH for high band and a bypass capacitor C3 which is grounded at its other end. An output terminal of high-band coil L4 on the hot side is connected to the next step through coupling capacitor C2. The output terminal of high-band coil L4 is also connected to a series circuit of a tuning capacitor C4 and a capacitance variable diode D1, which in turn is grounded. The cathode terminal of capacitance variable diode D1 is connected to a tuning voltage terminal VT through a tuning voltage applying resistor R1.
A series circuit of a switching diode D2 arranged in forward direction and a bias resistor R2 is connected from a junction of coil L3 and coil L4 to the ground while another series circuit of bias resistor R3 and bypass capacitor C5 is connected in parallel with bias resistor R2. Further, the earth side terminal of resistor R3 is connected to a power source terminal BL.
A junction between coil L1 and coil L2 on the earth side is connected with a coupling capacitor C6 to the cathode of diode D2. Disposed between the earth side terminal of earth side coil L1 and the output side terminal of hot side coil L3 is a damping resistor R4 which will be discussed later.
In this way, in the case of the VHF tuner, since it is impossible to cover the entire frequency range by varying only the capacitance of capacitance variable diode D1, a series of a high-band coil and a low-band coil is provided on each of the hot and earth sides while switching diode D2 for switching the bands is provided so that the activation and deactivation of the diode correspond to variations of inductance of the circuit.
In this configuration, when the low band is selected, a positive voltage will be applied to low-band power supply terminal BL and high-band power supply BH will be opened. Therefore, a proportionally allotted voltage produced by band changing bias resistors R2 and R3 is applied to the cathode of switching diode D2, so that the diode D2 becomes non-conductive.
FIG. 2A shows an equivalent circuit to the tuning circuit when the low band is selected. In the equivalent circuit, the earth side is formed by a series of coil L1 and L2 while the hot side is formed by a series of coil L3 and coil L4. Since switching diode D2 is non-conductive, damping resistor R4 is connected between the earth side terminal of earth side coil L1 and the output terminal of hot side coil L3 while a combined resistance Ra of bias resistors R2 and R3 is connected to damping resistor R4 in parallel with coil L2. A capacitance VC on the output side of coil L4 is a combined capacitance of tuning capacitor C4 and capacitance variable diode D1.
In this case, the effect of high-band coils L2, L4 is markedly low as compared to that of low-band coils L1, L3, so that the high-band coils can be neglected when low band is selected. Accordingly, the equivalent circuit at the time of the low band being selected may be considered as a single tuning circuit which, as shown in FIG. 2B, is composed of hot side low-band coil L3, earth side low-band coil L1 and capacitance VC with damping resistor R4 connected on the output side of coil L3.
When the high band is selected, a positive voltage will be applied to high-band power supply terminal BH and low-band power supply BL will be opened. Therefore, the diode D2 becomes conductive.
FIG. 3 shows an equivalent circuit to the tuning circuit at the time of the high band being selected. In the equivalent circuit, earth side low-band coil L1 and hot side low-band coil L3 are connected in parallel with one another with the help of coupling capacitor C6 and switching diode D2, forming a combined inductance La while bias resistors R2, R3 are denoted by a combined resistance Ra. Accordingly, a single tuning circuit is composed by hot side high-band coil L4, earth side high-band coil L2 and capacitance VC.
By the way, in a case of a tuning circuit in which an FET (field effect transistor) is used in the next step or the high-frequency amplifying circuit and at the same time switching between the low and high bands is to be made for selecting channel signals, the pain in the low band in general tends to be greater than that in the high band depending upon the characteristics of FET used. To deal with this, low-band damping resistor R4 is provided as shown in FIG. 1 in order to eliminate the gain difference. That is, this damping resistor R4 is effective as shown in FIG. 2B only when the low band is selected so that the gain of the tuning circuit is damped at the time of the low band being selected.
However, the thus configurated tuner contains many parts and therefore many connecting portions. Further, if all the parts are to be provided on a circuit board of a determined size, the packaging density becomes high, bringing about difficulty in soldering, and lowering the production yield. Therefore, the lowering of yield and the required costs for parts and labor due to the use of many parts heighten the production cost to a considerably high level.
To deal with this, the present applicant has previously proposed a tuner circuit as shown in FIG. 4. In this circuit, with no low-band damping resistor R4 provided, a coupling capacitor C6 is disposed between a junction of low-band coil L3 and high-band coil L4 on the hot side and a junction of bias resistors R2 and R3 while a switching diode D2 is provided between a junction point of low-band coil L1 and high-band coil L2 on the earth side and a junction of bias resistors R2 and R3. The remaining configuration of FIG. 4 is similar to that shown in FIG. 1. In other words, in the configuration shown in FIG. 1, switching diode D2 and coupling capacitor C 6 are exchanged with low-band damping resistor R4 being omitted.
In this arrangement, since, when the low band is selected, switching diode D2 becomes non-conductive, a single tuning circuit is composed by hot side low-band coil L3 and earth side low-band coil L1 and a capacitance VC as shown in an equivalent circuit in FIG. 5. In this case, a combined resistance Ra of bias resistors R2 and R3 functions as a damping resistance for the tuning circuit, so that damping resistor R4 is not required separately as used to be, the number of the parts and therefore the number of connecting portions can be reduced.
Nevertheless, the prior art tuner circuit described immediately above suffers a problem. That is, establishment of forward-directional current across switching diode D2 when the high band is selected, requires the resistance of bias resistor R2 to be lower than a certain level whereas establishment of a reverse-directional voltage applied across switching diode D2 when the low band is selected, requires a bias resistance ratio R2/R3 to be smaller than a certain value. These requirements limits the freedom of designing the circuit for determining the damping setup, and the like.